In the manufacture of objects of silicon nitride by sintering powder with isostatic pressing, the powder is suitably preformed into a manageable powder body prior to sintering and pressing. Preforming can be accomplished by subjecting the powder to a compaction, for example, by placing the powder in a sealed capsule of yielding material such as a plastic. The compaction can be conducted with advantage without the use of any binder, at a pressure of at least 100 MPa, and at room temperature or other temperature which is considerably below the temperature used during the compression in connection with the sintering. Thereafter, the preformed body can be given its desired shape by machining. Preforming can also be accomplished by, among other things, conventional techniques for the manufacture of ceramic goods. In these techniques, silicon nitride powder is typically mixed prior to preforming with a temporary binder, for example, with methyl cellulose, cellulose nitrate, an acrylate binder, a wax or a mixture of waxes. After preforming, the binder is driven off by heating so that the preformed powder body becomes essentially free from binder.
When the preformed powder body is subjected to isostatic pressing at the sintering temperature, it must, in order to give a desired dense, sintered product, be enclosed in a casing which is able to prevent the pressure medium used, normally a gas, from penetrating into the body during the pressing. The casing, which like its contents is liberated from undesirable gases during a processing step prior to sealing of the casing, must of course also have a sufficiently high strength or viscosity during pressing so as not to itself penetrate into the pores of the powder body. If a preformed capsule of glass is chosen as casing, the glass must be of a high-melting type in order not to run off or penetrate into the powder body at the high sintering temperature. However, when using glass, it is impossible to prevent the glass, when softening, from accumulating in pockets and other cavities of the preformed body. This accumulation often leads to fractures at protruding portions of the sintered article during cooling due to the differences in the coefficients of thermal expansion between silicon nitride and glass. Using a glass capsule in the method is therefore only suitable for the manufacture of objects having very simple shapes.
If it is desired to manufacture objects having very complicated shapes, the casing can be formed in situ by dipping the preformed powder body into a suspension of particles of high-melting glass or otherwise surrounding the body with a layer of particles of such glass and then heating the powder body under a vacuum at such a temperature that the particles form a dense casing about the body. This method permits the application of a casing which is thin and closely follows the shape of the powder body so that the tendency for accumulations of glass on the sintered object can be reduced, thus reducing the disadvantages connected therewith. It is also known to employ double layer of glass particles around the powder body, the innermost layer being of a high-melting glass and the outermost layer being of a low-melting glass.
After the powder body has been isostatically pressed, the glass casing must be removed from the body. Normally the removal is accomplished by blasting. However, during blasting, it may be difficult to avoid the occurrence of wounds or other damage on the surfaces of the silicon nitride body. As was mentioned above, damage caused by the existence of the glass casing may also occur during the cooling of the silicon nitride body from the sintering temperature.